Since low temperature poly-silicon (LTPS) which is used in liquid crystal display devices has high carrier mobility, it has become a technology widely used in current medium and small scale display devices. The development of an array process using LTPS technology as a base is also proceeding in organic EL display devices.
However, it is difficult to form a LTPS layer with sufficiently few variations in an excimer laser anneal process. Variation in the properties of a thin film transistor (TFT) due to variation in LPTS causes variation in luminosity of an organic EL display device.
Therefore, using both transistors manufactured by poly-crystal silicon which has high drive capabilities and also transistors using transparent amorphous oxide semiconductors in which low characteristic variation is expected as a measure for reducing power consumption and dealing with characteristic variation is being researched.
For example, a thin film transistor is disclosed in Japanese Laid Open Patent Publication No. 2015-056566 arranged with a transparent amorphous oxide semiconductor layer formed above a substrate, a gate insulation film formed above the transparent amorphous oxide semiconductor layer, a gate electrode formed above the gate insulation film, and a source electrode and drain electrode each formed so as not to overlap the gate electrode above the transparent amorphous oxide semiconductor layer, wherein the gate insulation film is processed to the same width as the gate electrode, and a resistance value of a region which does not overlap the gate insulation film in the transparent amorphous oxide semiconductor layer is lower than a resistance value of a region which overlaps the gate insulation film by a reduction process of a reduction gas.
However, the following problems exist in a display device arranged with a transistor which uses an oxide semiconductor. For example, when hydrogen or water infiltrates to an oxide semiconductor layer from a layer adjacent to the oxide semiconductor layer, there is concern that characteristics of the transistor will degrade. Specifically, there is a drop in mobility leading to variation in a threshold value and the like which can cause a variation in luminosity of the display device. On the other hand, it is known that the variation in characteristics of a transistor due to hydrogenation of a semiconductor layer is relieved in a transistor which uses poly-crystal silicon and the characteristics of the transistor are improved. In order to be able to respectively utilize the characteristics of a transistor which uses an oxide semiconductor and a transistor which uses poly-crystal silicon, it is necessary to overcome the conflicting properties caused by the effects of hydrogen and the like when actualizing a display device mounted with both transistors.